A conventional configuration for a communication network is one wherein a plurality of stations are interconnected by a common transmission medium or channel, and messages are exchanged among the stations on a time-shared basis over the channel. A so-called conflict situaton arises whenever multiple stations require use of the channel at the same instant.
For these contention periods, methods and concomitant arrangements exist for dealing with conflicts and these vary in complexity from simple, fixed priority techniques to sophisticated code division multiple access procedures. When the stations are arranged to sense a conflict on the channel, the type of communication network is referred to as a Carrier Sensed Multiple Access (CSMA) network.
As a specific example illustrative of the art for arbitrating contentions in a CSMA environment, the CSMA with Collision Detection (CSMA/CD) media access method is briefly described. (The CSMA/CD method is discussed in detail in the text entitled "Carrier Sensed Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications", published by The institute of Electrical and Electronic Engineers, Inc. in 1985). To transmit, each station monitors for a quiet period on the channel by detecting that no other stations are transmitting, and then emits the intended message in bit-serial form. If, after commencing transmission, the message collides with that of other stations, then each transmitting station intentionally sends additional bits to ensure propagation of the collision throughout the system. The stations then deactivate for a random time period, called the backoff time, before attempting to transmit again. This process continues until only a single message propagates collision-free over the channel. However, as is quite discernible even from this brief overview, this technique is somewhat inefficient due to station deactivation during the backoff periods and the randon nature of the retry process. From another viewpoint, if a normalized efficiency per transmission period is defined as the ratio of the time utilized for information-bearing message transfer to the total time period, then the efficiency is less than unity for conventional contention resolving systems.
Other, more specialized techniques have been disclosed to deal with contention on a more deterministic basis. Representative of these techniques is the disclosure in Great Britain Pat. No. 1,365,838. In this reference, the particular system considered is one in which a number of data handling devices (slave stations) are in communication with a common controller (master station) over a bus and any one of the devices may require service from the controller at any given time. At system initialization, each data handling device is assigned a fixed interrupt number indicative of the device address. The controller solicits interrupt requests by transmitting a coded message. Each device, if requesting use of the bus, responds by serially transmitting its interrupt number simultaneously with other devices. Then each device compares the bus content bit-by-bit and then either terminates or continues its transmission according to the results of the comparison. With such a technique, the time required to resolve a contention is fixed by the number of bits assigned to the interrupt numbers. However, even though this time is deterministic, there is still a period in which no actual information-bearing messages may be transmitted and the normalized efficiency is less than one.
As suggested by the above discussion, there have been many protocols introduced for dealing with conflicts in CSMA networks, and there have been numerous studies of their performance. With an offered load equal to the channel capacity, the throughput efficiency of all previous protocols is no more than 70 percent. Moreover, when the offered load exceeds a certain level in a given system, an overload condition referred to as "jamming" occurs, and the efficiency drops off, often significantly, above this threshold level.
None of the conventional techniques disclose or suggest a methodology for always achieving the optimum normalized throughput efficiency as defined above. Moreover, none of the conventional methods suggest the utilization of more than one channel or bus to resolve or even eliminate contention situations.